This is a result of stored gravitational potential energy seeking a path of release over friction.
Chemically, the equivalent to a snow avalanche is a spark causing a forest fire.
In 1913, the German chemist Max Bodenstein first put forth the idea of chemical chain reactions.
This means that one photon of light is responsible for the formation of as many as 106 molecules of the product HCl.
Nernst suggested that the photon dissociates a Cl2 molecule into two Cl atoms which each initiate a long chain of reaction steps forming HCl.
A. Christiansen and Hendrik Anthony Kramers, in an analysis of the formation of polymers, pointed out that such a chain reaction need not start with a molecule excited by light, but could also start with two molecules colliding violently due to thermal energy as previously proposed for initiation of chemical reactions by van' t Hoff.
A quantitative chain chemical reaction theory was created later on by Soviet physicist Nikolay Semyonov in 1934.
[3] Semyonov shared the Nobel Prize in 1956 with Sir Cyril Norman Hinshelwood, who independently developed many of the same quantitative concepts.
The reaction H2 + Br2 → 2 HBr proceeds by the following mechanism:[4][5] As can be explained using the steady-state approximation, the thermal reaction has an initial rate of fractional order (3/2), and a complete rate equation with a two-term denominator (mixed-order kinetics).
[4][5] The pyrolysis (thermal decomposition) of acetaldehyde, CH3CHO (g) → CH4 (g) + CO (g), proceeds via the Rice-Herzfeld mechanism:[7][8] The methyl and CHO groups are free radicals.
The sum of the two propagation steps corresponds to the overall reaction CH3CHO (g) → CH4 (g) + CO (g), catalyzed by a methyl radical •CH3.
This reaction is the only source of ethane (minor product) and it is concluded to be the main chain ending step.
Although this mechanism explains the principal products, there are others that are formed in a minor degree, such as acetone (CH3COCH3) and propanal (CH3CH2CHO).
Applying the Steady State Approximation for the intermediate species CH3(g) and CH3CO(g), the rate law for the formation of methane and the order of reaction are found:[7][5] The rate of formation of the product methane is
Szilárd knew of chemical chain reactions, and he had been reading about an energy-producing nuclear reaction involving high-energy protons bombarding lithium, demonstrated by John Cockcroft and Ernest Walton, in 1932.
In 1939, with Enrico Fermi, Szilárd proved this neutron-multiplying reaction in uranium.
An electron avalanche happens between two unconnected electrodes in a gas when an electric field exceeds a certain threshold.
Acceleration of these free electrons in a strong electric field causes them to gain energy, and when they impact other atoms, the energy causes release of new free electrons and ions (ionization), which fuels the same process.
If this process happens faster than it is naturally quenched by ions recombining, the new ions multiply in successive cycles until the gas breaks down into a plasma and current flows freely in a discharge.
Electron avalanches are essential to the dielectric breakdown process within gases.
The process can culminate in corona discharges, streamers, leaders, or in a spark or continuous electric arc that completely bridges the gap.
The process may extend huge sparks — streamers in lightning discharges propagate by formation of electron avalanches created in the high potential gradient ahead of the streamers' advancing tips.
Once begun, avalanches are often intensified by the creation of photoelectrons as a result of ultraviolet radiation emitted by the excited medium's atoms in the aft-tip region.
The extremely high temperature of the resulting plasma cracks the surrounding gas molecules and the free ions recombine to create new chemical compounds.
An avalanche breakdown process can happen in semiconductors, which in some ways conduct electricity analogously to a mildly ionized gas.
Semiconductors rely on free electrons knocked out of the crystal by thermal vibration for conduction.
This sets up conditions for the same type of positive feedback—heat from current flow causes temperature to rise, which increases charge carriers, lowering resistance, and causing more current to flow.
This can continue to the point of complete breakdown of normal resistance at a semiconductor junction, and failure of the device (this may be temporary or permanent depending on whether there is physical damage to the crystal).
Examples of chain reactions in living organisms include excitation of neurons in epilepsy and lipid peroxidation.
[10] A chain reaction in glutamatergic synapses is the cause of synchronous discharge in some epileptic seizures.